In the well known art of lithography a printing master or plate is employed having a printing surface on which the printing image areas are ink receptive whereas the non-printing background areas are water receptive. In the process of printing, an aqueous based so-called fountain solution is applied to the printing surface of the plate and such fountain solution adheres to the water receptive background areas only. An oil base ink is then applied to the printing surface of the plate. Such ink is repelled from the fountain solution containing background areas and adheres to the oil receptive printing image areas only. The printing plate is then brought into contact with paper on to which the image is printed by ink transfer from the printing image areas, as known in so-called direct lithography, or alternatively, as known in so-called offset lithography the printing plate is brought into contact with a rubber blanket on to which the image is offset by ink transfer from the printing image areas and in turn such rubber blanket is brought into contact with paper on to which the image is finally printed by ink transfer from the rubber blanket. The above described steps of applying the fountain solution and ink to the plate are repeated preparatory to each paper copy printing.
Lithographic printing plates can be prepared by numerous methods well known in the art, and one such method is electrostatic or electrophotographic imaging.
Typically in one electrostatic imaging process a lithographic printing plate is prepared by employing a conventional electrophotographic member for the formation thereon of a latent electrostatic image pattern by methods well known in the art and developing such image pattern by attraction thereto of electroscopic marking particles, followed by electrostatically transferring the thus formed image deposit on to a water receptive lithographic substrate or plate such as treated paper or grained aluminum and the like on which the transferred image deposit subsequently is fixed or fused and forms the ink receptive printing areas on the water receptive surface of the plate. The aforementioned electrophotographic member may comprise a selenium plate or a so-called binder plate consisting of a paper sheet having on one side thereof a coating of photoconductive zinc oxide contained within an insulating resinous binder material, as is well known in the art, or other inorganic or organic photoconductive layer disposed on a suitable backing or support material. The aforementioned electroscopic marking particles may comprise dry powder toners or so called liquid toners consisting of electroscopic particles dispersed in an insulating carrier liquid, as again is well known in the art.
A lithographic printing plate for off-set lithography is right way reading whereas for direct lithography the plate is wrong way reading. Thus to produce by electrostatic image transfer a right way reading lithographic plate for off-set lithography from an electrophotographic member such as a binder plate it is necessary to produce the image deposit on the binder plate either in wrong way reading sense and then transfer same directly on to the lithographic plate or to produce the image deposit on the binder plate in right way reading sense, then to transfer same on to an intermediate or off-set member and therefrom to transfer same to the lithographic plate.
In the instances where the image on the photoconductive member such as the binder plate is produced by exposure in contact with a transparency such as a film positive or negative it is necessary to have the silver halide emulsion side of the transparency in contact with the electrostatically charged surface of the binder plate in order that no loss in resolution occurs. Thus to obtain a right way reading image on the binder plate the transparency needs to be right way reading emulsion side down whereas for a wrong way reading image on the binder plate the transparency needs to be wrong way reading emulsion side down.
Conventional non-electrostatic lithographic plate making processes generally require the transparency to be right way reading emulsion side down for off-set lithography and wrong way reading emulsion side down for direct lithography. Thus positive and negative film transparencies are made emulsion side down in right or wrong way reading sense depending on their final purpose, and accordingly there is need for a method of and means for employing film transparencies regardless of the sense in which they are produced for the preparation by electrostatic image transfer as described in the foregoing of lithographic printing plates in right or wrong way reading sense, as may be desired.
It will be realised that if irrespective of the sense in which it is prepared a transparency is contact exposed emulsion side down with an electrophotographic member, the image deposit formed on such member can produce by electrostatic transfer a lithographic printing plate in any desired sense depending whether the electrostatic transfer of such image deposit is carried out directly or by offset means. Accordingly there is need for a method of effecting by one and the same means optionally, as may be desired from case to case, direct or offset electrostatic transfer of image deposit from an electrophotographic member on to a lithographic plate.
Electrostatic direct transfer methods are known and electrostatic offset transfer methods are also known. In U.S. Pat. No. 3,862,848 there is disclosed a method of electrostatic offset transfer of colour images, however the means described in this patent require both the electrophotographic or dielectric member and the image receiving member to be flexible and are thus not applicable to the production of lithographic printing plates in those instances in which the printing plate is a relatively rigid metal sheet. It is therefore the general object of this invention to provide a novel method of and means for effecting by one and the same means optionally direct or offset electrostatic transfer of image deposit from an electrophotographic or dielectric member on to a rigid or flexible transfer receiving member.
Another object of this invention is to provide a novel method of and means for effecting by one and the same means optionally direct or offset electrostatic transfer of image deposit from an electrophotographic or dielectric member onto lithographic printing plate material.
The foregoing objects and other advantages are accomplished in accordance with this invention in the manner as described in the following.